1. Field of the Invention
This invention relates generally to an apparatus for detecting arcing faults, and more specifically, to such an apparatus for detecting arcing faults on low-voltage spot networks.
2. Description of the Prior Art
Low-voltage electrical power networks consist of interlaced loops or grid systems. Electrical energy is supplied to the network by two or more power sources, so that the loss of any one source does not result in an interruption of electrical service. These systems provide the highest level of reliability possible with conventional power distribution and are normally used to serve high-density load areas. Primary applications are in central or downtown city areas, large buildings, shopping centers, and some industrial plants. Network systems are of either the grid-type or spot-type. The electrical service is three-phase, three-wire or three-phase, four-wire at 208Y/120V or 480Y/277V.
In a grid system, the loads and sources form a grid pattern. Several sources, each usually employing a dedicated feeder, supply electrical power to the network. The source is connected to the network via a high voltage switch, a three-phase network transformer, and a network protector. The transformer secondary is usually 208Y/120V or 480Y/277V wye-connected, for three-phase, four-wire service to the network through the network protector. The network protector consists of an air circuit breaker, and normally a network master relay. When a source of supply or primary feeder is lost, the load formerly supplied by that feeder is carried by the remaining feeders.
Large concentrated load areas, such as commercial buildings and shopping centers, are frequently served by spot networks. Spot networks consist of two or more network units fed by two or more primary feeders. Typically, the spot network primary cables are tapped from non-dedicated feeders.
Typical low-voltage spot network installations operated at nominal 480Y/277V (line-to-line voltage grounded wye configuration) are not protected against network faults. Normally, a 480V bus is supplied by multiple transformer installations connected to two or more high-voltage primary circuits. Between the transformer and the 480V bus work is a network protector. The network protector isolates the transformer from the bus in the event of a fault in the transformer or the primary circuit feeding it. High reliability is achieved since primary faults are isolated and the network is carried by the other feeders connected to it.
The network protector also opens when a fault in the primary feeder would cause power flow from the network to the feeder, that is, reverse power flow. The network protector is not designed to open for faults on the network itself. In turn, fuses in the network are designed with a long time delay to act as back-up protection for the network protector for primary system faults. In addition, the master relay of the network protector opens the circuit breaker when the primary feeder is disconnected from its source of supply and magnetizing current flows from the secondary network into the network transformer. To summarize, the circuit breaker opens when the network transformer associated with it is not delivering power to the network, and when conditions are such that total three-phase power would flow from the network into the primary feeder. To protect against reverse power flow, the network master relay monitors total three-phase power direction. Typical spot network installations do not contain phase overcurrent or ground overcurrent protective relays or any form of overcurrent protection for the network, other than the fuses.
Phase-to-ground or phase-to-phase faults that start in the 480V bus work due to contamination, human error, etc. normally involve a power arc. The voltage produced across the arc can limit the fault current to values less than the rating of the network fuses in the bus work and can typically be on the order of the load current. Under such conditions, the network fuses will not open. However, these power arcs represent a tremendous concentration of energy at the point of the arc and the heat released represents a hazard and can destroy 480V bus work.